A ceramic sintered body of interest to this invention is obtained by forming a low-temperature cofired ceramic (LTCC: Low Temperature Cofired Ceramic) material into a predetermined shape, and firing the material.
The low-temperature cofired ceramic material can be fired simultaneously with low melting point metal materials such as silver and copper, which are relatively low in specific resistance, thus form multilayer ceramic substrates which have excellent high frequency characteristics, and have been frequently used as, for example, a substrate material for high-frequency modules in information-communication terminals.
So-called glass-ceramic composite systems are common as the low-temperature cofired ceramic in which a B2O3—SiO2 based glass material is mixed with a ceramic material such as Al2O3. However, these systems require the use of a relatively expensive glass material as a starting raw material, and contain boron which is likely to volatilize during firing, and the compositions of substrates obtained are thus likely to vary. Therefore, the production processes have complications, such as the need to use a special setter for controlling the volatilization volume of boron.
Low-temperature cofired ceramic materials have been proposed which are disclosed in, for example, Japanese Patent Application Laid-Open No. 2002-173362 (Patent Document 1), Japanese Patent Application Laid-Open No. 2008-044829 (Patent Document 2), Japanese Patent Application Laid-Open No. 2008-053525 (Patent Document 3), and International Publication WO 2009/025156 (Patent Document 4). The low-temperature cofired ceramic materials disclosed in these references, for which the starting raw materials contain no glass, are moreover non-glass low-temperature cofired ceramic materials containing no boron, and thus, do not encounter the problem described above.
However, the ceramic sintered bodies obtained by sintering of the low-temperature cofired ceramic materials disclosed in these references may fail to achieve desirable strength properties, because of their small fracture toughness values.